The 25 most dangerously polluted cities in the US

More than 125 million Americans live in counties with unhealthy levels of air pollution, according to the American Lung Association's 2017 "State of the Air" report.

Pollution leads to various kinds of nasty health outcomes, including asthma, lung cancer, and shortened lifespans.

The report, released April 19, rounds up air quality data from 2013, 2014, and 2015 in the United States. (Because air quality data is plentiful and complicated, the annual ALA reports usually look two years into the past.)

The report focuses on two key kinds of pollution: particles and ozone.

Particles include everything from dust kicked up during a drought to tiny particles floating in the air from forest fires or fossil fuels. Ozone, or smog, develops in the upper atmosphere when emissions from tailpipes and smokestacks cook in the sun's heat.

Local pollution levels can be measured according to amounts of ozone in the atmosphere, number of days with extremely high particle pollution levels, or levels of year-round particle pollution. Each of those metrics yields different results — Los Angeles is the worst US city for ozone, for example, but not when it comes to year-round particle pollution, which is the ranking we use in this article.

Janice Nolen, the lead author of the ALA report, told Business Insider that local problems like car traffic can make cities' ozone or particle problems worse, but that the overall issue is a national one. Even a place with strong environmental rules can be vulnerable to bad air flowing in from other parts of the country.

Here are the US cities with the worst year-round particle pollution. (The report notes, however, that data is missing on all of Illinois, most of Tennessee, and part of Maine.) For more information and suggested policy solutions, you can read the full report.